Deposition apparatus

ABSTRACT

Deposition apparatus incorporating either a single or multiple filtered cathodic arc (FCA) source for depositing coatings such as tetrahedral amorphous carbon (TAC); metal oxides; compounds and alloys of such materials onto various types of substrates, such as metals semiconductors, plastics ceramics and glasses. Substrates are moved through the plasma beam(s) of the FCA source(s) and beam scanning increases deposition area. Macroparticles are filtered by a double bend filter duct.

[0001] The present invention relates to deposition apparatus.Specifically, the invention relates to apparatus and processes fordeposition of optical quality coatings such as: tetrahedral amorphouscarbon (TAC); metal oxides, nitrides, hydrides, carbon-containingcompounds; and other compounds and alloys of metals.

[0002] Various methods are known in the prior art for depositingthin-films on substrates. In the field of physical vapour deposition,with which the invention is concerned, these methods include varioussputtering techniques such as RF or magnetron sputtering, and the use offiltered cathodic arc sources of positive ions.

[0003] U.S. Pat. No. 4,851,095 describes apparatus and process formagnetron sputtering to obtain thin coatings on a range of substrates.Magnetron sputtering can produce a broad beam of coating particles andis thus a suitable technique for the coating of large substrate areas.To date, the films produced by magnetron sputtering are not ofsufficient quality in terms of hardness, uniformity and smoothness to besuitable for commercial production of coatings for optical equipment.U.S. Pat. No. 4,851,095 also describes a coating chamber thatincorporates a rotatable drum on which is mounted substrates to becoated.

[0004] Deposition of coatings using a filtered cathodic arc source isalso known in the art, and reviewed by P. J. Martin in SurfaceEngineering, Volume 9, (1993) no. 1, pages 51-57.

[0005] Filtered cathodic arcs known in the art are typically used forshort periods of time, or in pulsed and non-continuous mode for coatingindividual substrates one at time. The problems associated withcommercial use of this technology have not been solved. Further, knownfiltered cathodic arc sources typically produce a plasma beam no morethan 3 cm in diameter. This is not a suitable size for coating largesubstrate areas.

[0006] Neither known physical vapour deposition or chemical vapourdeposition techniques have previously been considered suitable for useon a commercial scale.

[0007] There has thus been appreciated the need to be able to deposithigh quality thin films on substrates in a commercial process usingapparatus that can be used continuously for relatively long periods oftime.

[0008] It is an object of the invention to provide deposition apparatusfor commercial deposition of high quality thin films onto substrates. Itis another object of the invention to provide deposition apparatus fordeposition of high quality thin films onto large substrate areas. Afurther object of the invention is to provide deposition apparatus forapplying multi-layer coatings onto substrates.

[0009] These objects are achieved at least in part by the combination ofa coating chamber and a filtered cathodic arc source.

[0010] According to the invention there is provided apparatus forapplying a coating of positive ions to a substrate comprising:

[0011] a vacuum chamber,

[0012] a filtered cathodic arc source providing a plasma beam containingthe positive ions,

[0013] a substrate to be coated, and

[0014] a substrate holder,

[0015] wherein the substrate holder is adapted to move the substrateacross the beam of positive ions thereby to coat the substrate with thepositive ions.

[0016] In an embodiment of the invention, the apparatus comprisesmagnetic means for scanning the plasma beam over a coating area greaterthan the area of the plasma beam.

[0017] The invention thus enables a large area of substrate to be coatedas the substrate is moved through the beam of positive ions generated bythe filtered cathodic arc. This enables efficient commercial scaledeposition of positive ions onto substrates. By scanning the plasma beaninto a coating area of greater size than the area of the plasma beamemitted from the filtered cathodic arc, coating of substrate over anespecially large surface area is enabled. Further, the coating beam isnecessarily of lower density than the smaller area plasma beam prior toscanning. Therefore, the deposition rate of the scanned beam is lowerthan the deposition rate of an unscanned beam and deposition on thesubstrate occurs more slowly. This enables greater control over thedepth of deposition on the substrate.

[0018] In an embodiment of the invention, the substrate holder isadapted for rotation of the substrate through the plasma beam. In apreferred embodiment, the substrate holder is a rotatable drum and thesubstrate is mounted on the inner or outer periphery of the drum.

[0019] In use of this embodiment immediately above described, one or aplurality of substrates are mounted on the drum periphery and byrotation of the drum while a plasma beam is generated from the filteredcathodic arc a layer of positive ions is deposited onto each substratein turn as it passes through the plasma beam, which is preferablyscanned into a coating beam. The rate of deposition of positive ionsonto the substrates is conveniently monitored using techniques known inthe art, for example using a crystal rate monitoring system.

[0020] In a particular embodiment of the invention the rate ofdeposition on the substrate is monitored and this deposition informationis fed back into the magnetic beam scanning apparatus so as to controlthe rate of deposition on any particular area of the substrate.

[0021] In a further embodiment of the invention, scanning of the plasmabeam occurs downstream of filtering of the plasma beam by the filteredcathodic arc source. In another embodiment of the invention the magneticscanning means scans the plasma beam in a raster scan. The width of theraster is preferably at least 10 cm wide, more preferably at least 20 cmwide and most preferably at least 30 cm wide.

[0022] The apparatus of the invention uses a filtered cathodic arcsource for continuous coating of one or a plurality of substrates withpositive ions from a target at the cathode of the cathode arc source.This is made possible by the filtered cathodic arc source being suitablefor continuous use without overheating. This can be achieved by theprovision of a water-cooled anode and a water-cooled cathode in thefiltered cathodic arc source, making the source suitable for continuousor long term use. Typically, for a filtered cathodic arc source,continuous use means use for a period of at least 3 minutes, but canalso mean use until the target, located at the cathode and from whichthe positive ions in the coating beam are generated, is substantiallyconsumed. As will be appreciated by a person of skill in the art,complete consumption of the target material is rare as contamination ofthe plasma beam by ions generated from the cathode material ispreferably to be avoided; in practice the target is generally notallowed to be consumed beyond a point at which there is a risk ofcontaminating the plasma by arcing directly between cathode and anode.

[0023] A particularly preferred filtered cathodic arc source for use inthe apparatus of the invention is described and claimed in a co-pendingInternational patent application filed in the name of Avimo SingaporeLimited on the same day as the present application, the content of whichis incorporated herein by reference.

[0024] In a particularly preferred embodiment of the invention suitablefor applying multi-layer coatings of positive ions to a substrate, theapparatus further comprises at least a second filtered cathodic arcsource providing a plasma beam containing positive ions, and a substrateholder adapted to move the substrate across the beams from therespective filtered cathodic arc sources.

[0025] In use of an embodiment of the invention, a first filteredcathodic arc source is used to place a first coating layer on asubstrate; subsequently, this first cathodic arc source is stopped and asecond cathodic arc source is used to place a second layer of adifferent material onto the substrate. This technique is advantageouslyused to deposit multi-layer coatings onto optical elements. Amulti-layer coating made using the invention comprises a first layer oftetrahedral amorphous carbon, a second layer of silicon dioxide, a thirdlayer of tetrahedral amorphous carbon and a fourth layer of silicondioxide. Another optical coating obtainable using the apparatus of theinvention has a first layer of aluminum oxide, a second layer oftetrahedral amorphous carbon and a third layer of silicon dioxide. Othercombinations of coatings in multi-layer coatings will be apparent to thepractitioner.

[0026] Multi-layer coatings are also achieved in another embodiment ofthe invention in which deposition apparatus comprises a filteredcathodic arc source and the source has at least two cathode targets, onein a vacuum chamber at a cathode station from which an arc can begenerated and the other or others stored away from the cathode station.The cathode targets are interchangeable without breaking vacuum in thechamber. Means for interchanging the cathode targets convenientlycomprises a cathode gripper mounted on an arm and moveable between thecathode station and the stored cathode target or targets, which may bein a cathode magazine.

[0027] Scanning of the plasma beam can be achieved by scanningtechniques known and appreciated in the art. For example, a magneticscanner using a soft magnetic core can be used with a scanning frequencyof 2-100 Hz.

[0028] In a particular embodiment of the invention, the plasma beam isscanned in two dimensions using magnetic fields scanned perpendicular toeach other. This scanning technique produces a coating beam that is twodimensional in area and can be used in the coating apparatus of theinvention. By monitoring the rate of deposition on the substrate andactively feeding back this deposition rate information to the scanningapparatus coatings of excellent uniformity are achieved. An embodimentof the invention uses electronic scanning control means. This controlmeans is pre-programmed with a pre-determined rate of deposition forseparate areas of the substrate. The rate of deposition in each area ofthe substrate is monitored, this information is fed back to the scanningcontrol means and the scanning is adjusted so as to obtain the desiredrate of deposition in each deposition area.

[0029] In embodiments of the invention, when carrying out scanning ofthe beam in two dimensions, a typical arrangement is a scanningfrequency of about 50 Hz in one dimension and about 2 Hz in the otherdimension. As will be appreciated by a person of skill in the art,scanning of the beam using frequencies that are chosen so that thefrequency in one dimension is significantly different to the frequencyin the other dimension gives a paint brush type scanning pattern. Whenfrequencies are chosen so as to be similar in both dimensions, theresult is typically a pronounced scanning pattern. Synchronised scanningfrequencies in both dimensions produces Lissajou figures.

[0030] In typical use of deposition apparatus according to theinvention, the plasma beam in scanned rapidly in one dimension only, ata scanning frequency typically of at least 20 Hz, preferably around40-80 Hz. A convenient frequency is that of mains electricity, namelyaround 50 Hz. Scanning in one dimension is combined with movement of thesubstrate across the scanned beam, a substrate being mounted on theperiphery of a rotating drum. A typical rotation produces substratemovement in the range of 0.1 m/s to 10 m/s. It is convenient to arrangethe scanning frequency so as to be approximately 10 times that of therotating drum.

[0031] It is particularly preferred to scan the plasma beam in onedimension only. This enables the dimensions of the duct containing thescanned plasma beam to be fan-shaped and enables the scanning mechanismto be approached close to the plasma beam inside the plasma duct.

[0032] The use of a scanned plasma beam confers considerable advantagesover prior art magnetron sputtering devices as these devices generateatoms of coating material which are neutrally charged and cannot bescanned. The scanning mechanism of the preferred embodiment of theinvention enables precise user control of the rate of deposition on thesubstrate as well as precise control over the deposition profile for thesurface of a substrate.

[0033] In another embodiment of the invention the apparatus comprisesmeans for biasing the substrate to a pre-determined positive potential.Preferably, the substrate is biased to a potential within a certainrange, preferably to within 10V-30V. It is preferred to control thepositive bias on the substrate to control the energy of positive ionsarriving at the substrate and thereby to alter the properties of thedeposited layer.

[0034] For optical elements and other dielectric substrates, RF meanscan be used to provide the appropriate biasing.

[0035] Using apparatus according to the invention it is possible todeposit high quality films of tetrahedral amorphous carbon from agraphite target. It is known that such high-quality films are typicallycompressively stressed. In a preferred embodiment of the invention asubstrate is coated with both a layer of tetrahedral amorphous carbonthat is compressively stressed and also a layer of a coating that istensile stressed, such as aluminium oxide (Al₂O₃), zinc sulphide (ZnS₂)or zinc selenide (ZnSe₂).

[0036] In a particularly preferred embodiment described in detail below,the invention uses a cylindrical processing configuration in whichsubstrates are mounted on a rotating cylindrical drum carrier. Thesubstrates are moved past a set of processing stations comprisingfiltered cathode arc (FCA) sources alternately to deposit singlethin-film layers of a particular material which culminates in a completemulti-layer thin-film system.

[0037] The substrates are optionally electrically biased with a DC or RFsystem that confers an acceleration potential for the discharged ionsemerging from the FCA source. Also, this arrangement is scalable in thata multiple number of FCA sources can be installed to deposit over alarger substrate area. As one example, three FCA sources are arranged tosequentially deposit TAC, Aluminum Oxide, and Silicon Dioxide to form ananti-reflective coating on either plastic or glass substrates. Asanother example two or more FCA sources are adapted to operatesimultaneously with targets of the same material to deposit a coating ona substrate, or plurality of substrates, at increased rate.

[0038] Another aspect of the invention provides apparatus for applying acoating of positive ions onto a dielectric substrate, the apparatuscomprising:

[0039] means for generating an arc at a cathode target, the cathodetarget containing the ions to be deposited on the dielectric substrate,

[0040] magnetic means for directing a beam of ions emitted from thecathode along a filter path substantially to remove macroparticlestherefrom,

[0041] means for holding the dielectric substrate in the filtered ionbeam, and

[0042] means for applying RF bias to the optical element to dissipateelectrostatic charge accruing on the element by deposition of positiveions.

[0043] The apparatus of the invention, incorporating the FCA depositionsources provides rapid, uniform deposition of optical quality coatingson both flat and curved parts. The efficiency of the FCA depositionsystem provides high deposition rates coupled with the thermal anddeposit spread over a large substrate surface area permitting high rateof thin-film synthesis on plastic and other temperature-sensitivematerials.

[0044] By adjusting the starting material, deposition rate, reactive gastype, and substrate bias we can deposit films consisting of precisestoichiometry and composition.

[0045] A particular aspect of this invention, as compared to othersystems, is the ability to deposit tetrahedral amorphous carbon (TAC),or “diamond like carbon” (DLC), on ambient temperature substrates. Thequality of this film exceeds other DLC films deposited at elevatedtemperature in terms of hardness, thermal conductivity, refractiveindex, and surface smoothness. TAC in combination with other materialsprovides a unique technology for depositing multi-layer thin-films ontoa variety of shapes and temperature-sensitive substrates.

[0046] Specific embodiments of the invention combine FCA depositionsources and a rotary cylindrical work-piece support to provide adeposition system which is capable of high rate synthesis of single ormulti-layer optical films of materials such as, but not limited to,tetrahedral amorphous carbon (TAC), Al₂O₃, TiO₂, Ta₂O₅, and SiO₂. Thiscombination provides for high rate depositions of optical qualitythin-films on temperature-sensitive substrates. The application of a DCor RF bias (for conductive and non-conductive substrates, respectively)is necessary to the deposition of the films. This DC or RF bias isapplied to the substrate holder which is electrically isolated from thechamber.

[0047] There now follows a description of specific embodiments of theinvention in which:

[0048]FIGS. 1 and 2 are, respectively, a simplified schematicperspective view and a simplified schematic horizontal sectional view,of a single-rotational cylindrical drum FCA vacuum coating system whichembodies the principles of our present invention;

[0049] FIGS. 3-8 depict calculated the transmission or reflectancecurves for a single layer of TAC deposited on a Germanium substrate(FIG. 3), a multi-layer anti-reflection coating on Germanium consistingof TAC and Ge (FIG. 4), a multi-layer anti-reflection coating on eitherglass or plastic substrates consisting of TAC and SiO₂ (FIG. 5), amulti-layer, anti-reflection coating on either glass or plasticsubstrates consisting of TAC, Al₂O₃, and SiO₂ (FIG. 6), a transmissioncurve of multi-layer laser mirror coating consisting of TAC and SiO₂(FIG. 7), and a reflection curve of the same multi-layer laser mirrorcoating consisting of TAC and SiO₂ (FIG. 8); and

[0050]FIGS. 9 and 10 are photographs of deposition apparatus accordingto the invention.

[0051]FIGS. 1 and 2, respectively, depict a simplified schematicperspective view and a horizontal sectional view of a single rotationembodiment of our FCA vacuum system. The illustrated FCA system (10)comprises a housing (111) which forms a vacuum processing chamber and isconnected to a suitable vacuum pumping system (19) shown in FIG. 2. Thevacuum pumping system includes a suitable vacuum pump or combinationsthereof for exhausting and pumping down the vacuum chamber to a vacuumof at least 10⁻⁶ Torr and a mechanical pump for vacuum regeneration, asis conventional with vacuum chambers. The chamber is mounted on a framecomprising a rack for instruments and a control panel having electricalconnections to sensors in and around the chamber to monitor chamberpressure, arc power supply, drum motor power supply and deposition rate.The system (10) also includes a cage-like drum (15) which is mounted forrotation about shaft (13) and has a cylindrical side which is adaptedfor mounting substrates (16) of various configurations. The substrates(16) can be mounted directly on the drum (15) facing outwardly towardthe FCA sources (1 2) and the linear ion source (17) which are spacedabout the external periphery of the drum (15).

[0052] The rotating drum (15) is located within a vacuum chamber that isroughly cylindrical and has a port allowing access to a cylindrical drummounted within the chamber. The chamber diameter is approximately 24inches (50 cm) and the drum diameter is approximately 15 inches (38 cm).The drum is approximately 15 inches (38 cm) high.

[0053] The drum is rotated via a vacuum rotary feed-through, allowingrotational drive to be imparted to the drum without breaking vacuum inthe chamber. An electric motor (not shown) is located on top of thechamber to drive rotation of the drum via a central shaft attached tothe drum and projecting out of the chamber through the vacuum rotaryfeed-through. The drum is mounted on bearings on the bottom of thechamber.

[0054] Three filtered cathode arc sources (12) are attached to thevacuum chamber at distal ends of their respective plasma ducts, thesedistal ends meeting the vacuum chamber wall at rectangular openingsapproximately 7 inches wide and 12 inches high (18 cm wide and 30 cmhigh). In both cases, the distal plasma ducts of the filtered cathodearc sources are attached to the wall of the chamber such that when noplasma beam scanning is taking place the plasma beam is normal to thechamber wall and normal to the cylindrical drum. The filtered cathodearc sources comprise double bend plasma ducts for filteringmacroparticles from the plasma beam. The plasma ducts are approximately6 inches (15 cm) in diameter, increasing at their distal ends to arectangle of 7 inches wide and 12 inches high (18 cm wide and 30 cmhigh) to allow for scanning of the beams.

[0055] The cathode arc sources (12) are located, when viewed from aboveapproximately 90 degrees apart and operate independently of each other.Each can provide a deposition rate of 15 angstroms per second over anarea of 25 in² (157 cm²) at an arc current of 70A.

[0056] Each filtered cathode arc source has a water cooled anode and awater cooled cathode, and also water cooling for the coils providingmagnetic steering fields for the double bends. Each filtered cathode arcsource can operate continuously, i.e. until the cathode target issubstantially consumed.

[0057] A deposition rate monitor (not shown) is located inside the drumand attached to a central rod inside the drum, and having electricalconnections via the vacuum feed-through to the control panel. Aperturesin the surface of the cylindrical drum allow ions from the plasma beamto impinge upon the deposition rate monitor and allow accuratemonitoring of deposition rate.

[0058] The drum is electrically insulated from the chamber and theapparatus allows the operator the option of applying a DC bias or an RFbias to the drum. The apparatus allows a bias of up to 1000 volts to beapplied to the drum, applied through the drum shaft.

[0059] In use, the filtered cathode arc sources can be usedsimultaneously to obtain high deposition rates of diamond-like carbonfilms having low macroparticle counts. While the deposition apparatus ofthis specific embodiment uses three filtered cathode arc sources, itwould be a matter of routine for a person of skill in the art to prepareapparatus having only one, or two or more than two filtered cathode arcsources. Of practical concern, an additional source can be located onthe door of the vacuum chamber.

[0060] The deposition apparatus of the specific embodiment is also usedto deposit multi-layer coatings on to a substrate without breakingvacuum in the vacuum chamber. To obtain such multi-layer coatings, onefiltered cathode arc source is operated using a target such as graphite,producing a diamond-like carbon first layer, and the second filteredcathode arc source is not operated during deposition of the first layerbut is thereafter operated using a different target, such as siliconwith injection of oxygen gas to obtain a silica layer on top of thediamond-like carbon layer. The calculated reflectance of such amulti-layer coating is shown in FIG. 5.

[0061] Each filtered cathode arc source has a filter duct between thecathode target and the substrate, and a magnetic steering field providedby coil windings around the duct to steer positive ions through theduct.

[0062] The invention is of use in providing the following coatings:

[0063] 1. Germanium and Silicon for infrared Applications: Rain ErosionProtective Multi-layer Coatings.

[0064] 2. Zinc Sulphide and Zinc Selenide for Infrared Applications.

[0065] 3. Glass Eyeglass Lenses.

[0066] 4. Plastic Eyeglass Lenses.

[0067] 5. Anti-Reflective Coatings for Plastics.

[0068] 6. Multi-Layer Designs for a Variety of Substrates.

[0069] 7. Tool Hardening Applications.

[0070] 8. Protective Coating for Hard Disk Drives.

[0071] 9. Medical Applications: Durable, frictionless Artificial Joints.

[0072] 10. Hard Smooth Coatings for Tape Reading Heads.

[0073] 11. Car Panels, Windscreens.

[0074] Referring to FIGS. 9 and 10, deposition apparatus (100) comprisesa vacuum chamber (120) and two FCA sources (102, 111). The first source(102) has a cathode and anode (101) for generating an arc from a target(not shown). Positive ions from the target are filtered by a double bendduct comprising a first straight section (103), a first bend (104), asecond straight section (105), a second bend (106) and a third straightsection (107) that opens into the vacuum coating chamber (120). Bothducts are toroidal in cross-section and have a double bend preventing aline-of-sight from the target to the substrate and preventing also asingle bounce path from the target to the substrate. Ports (121) on thechamber allow visual inspection of substrates mounted on a rotatabledrum (not shown) inside. Positive ions are steered through the duct by amagnetic field produced from coil windings around the whole length ofthe duct.

[0075] The first bend (104) has an angle of 50 degrees and the secondbend (106) has an angle of 60 degrees. These two bends are in differentplanes, such that the resultant angle between (i) plasma entering theduct and passing through the first straight section (103) and (ii)plasma passing through the third straight section and exiting the ductis 90 degrees.

[0076] Likewise, positive ions from the second source are filtered by adouble bend duct having first, second and third sections (112, 114, 116)and first and second bends (113, 115). In the case of the second source,the first bend (113) has an angle of 35 degrees, the second bend (115)has an angle of 40 degrees and the resultant angle between the first andthird straight sections (112, 116) is 45 degrees.

[0077] A frame (122) bears the coating chamber and the two FCA sources.Other aspects of the apparatus as described for FIGS. 1 and 2.

[0078] Variations and modifications from the described specificembodiments will be apparent from the description to a person of skillin the art and consequently the invention is not to be construed aslimited to any specific embodiment.

1. Apparatus for applying a coating of positive ions to a substratecomprising: a vacuum chamber, a filtered cathodic arc source forproviding a plasma beam containing the positive ions, a substrate to becoated, and a substrate holder, wherein the substrate holder is adaptedto move the substrate across the beam of positive ions thereby to coatthe substrate with the positive ions.
 2. Apparatus according to claim 1comprising magnetic means for scanning the plasma beam over a coatingarea greater that the area of the plasma beam.
 3. Apparatus according toclaim 2 wherein scanning of the plasma beam occurs downstream of beamfiltering.
 4. Apparatus according to claim 2 or 3 comprising means forscanning the plasma beam in a raster.
 5. Apparatus according to claim 2, 3 or 4 wherein the beam is scanned to a width of at least 10 cm,preferably at least 20 cm, most preferably at least 30 cm.
 6. Apparatusaccording to any of claims 1-5 wherein the substrate holder is adaptedfor rotation of the substrate through the plasma beam.
 7. Apparatusaccording to claim 6 wherein the substrate holder is a rotatable drumand the substrate is mounted on the inner or outer periphery of thedrum.
 8. Apparatus according to any of claims 1-7 for applyingbmulti-layer coatings of positive ions to a substrate, the apparatusfurther comprising at least a second filtered cathodic arc sourceproviding a plasma beam containing positive ions, and a substrate holderadapted to move the substrate across the beams from the respectivefiltered cathodic arc sources.
 9. Apparatus according to any of claims1-8 wherein a filtered cathodic arc source comprises a first cathodelocated at a cathode station, means for generating an arc at thestation, a second cathode and means for interchanging the cathodeswithout breaking vacuum.
 10. Apparatus according to claim 9 wherein thesecond cathode is one of a plurality of cathodes stored in a cathodemagazine.
 11. Apparatus according to any of claims 1-10 comprising meansfor applying a DC or RF bias to the substrate.
 12. Apparatus accordingto any preceding claim comprising a filtered cathode arc sourcecomprising a filter duct having two bends.
 13. Apparatus according toclaim 12 in which the filter duct has a first bend in a first plane anda second bend in a second plane that is not co-incident with the firstplane.
 14. Apparatus according to any preceding claim comprising twofiltered cathode arc sources having cathode targets of the same materialand adapted for simultaneous operation.
 15. Apparatus according to anyof claims 1-14 for coating positive ions onto a dielectric opticalsubstrate.
 16. Apparatus for applying a coating of positive ions onto adielectric substrate, the apparatus comprising: means for generating anarc at a cathode target, the cathode target containing the ions to bedeposited on the dielectric substrate, magnetic means for directing abeam of ions emitted from the cathode along a filter path substantiallyto remove macroparticles therefrom, means for holding the dielectricsubstrate in the filtered ion beam, and means for applying RF bias tothe optical element to dissipate electrostatic charge accruing on theelement by deposition of positive ions.
 17. Apparatus according to claim16 comprising a filter duct having two bends.
 18. Apparatus according toclaim 17 wherein each bend is at least 20 degrees and the bends are innon-coincident planes.